Carbon fibers are lightweight and also have excellent strength, rigidity, dimensional stability and the like. Accordingly, composite materials combined with various matrix resins have been used in a large number of fields, including airplane components, spacecraft components, automobile components, ship components, civil engineering and construction materials, sporting goods, and the like. In a carbon fiber reinforced composite material using a carbon fiber, it is important that adhesion between the carbon fiber and the matrix resin is excellent in order to take advantage of excellent characteristics of the carbon fiber.
In order to improve the adhesion between a carbon fiber and a matrix resin, usually, a method in which the carbon fiber is subjected to an oxidation treatment such as gas-phase oxidation or liquid-phase oxidation and thereby introducing oxygen-containing functional groups into the carbon fiber surface, has been performed. For example, a method in which a carbon fiber is subjected to an electrolytic treatment and thereby improving the interlaminar shear strength, which is an index of adhesion, has been proposed (Patent Document 1). However, in recent years, as the level of required characteristics to a composite material is improved, the adhesion achievable only by such oxidation treatment is being insufficient.
Particularly, in a carbon fiber having few functional groups and a molded article of a carbon fiber reinforced composite material using a matrix resin, a sizing agent to be applied to the carbon fiber has been studied, for the purpose of improving interfacial adhesion between the carbon fiber and the matrix resin. As such a sizing agent, a phenolic resin, a melamine resin, a bismaleimide resin, an unsaturated polyester resin, an epoxy resin or the like has been preferably used. Among them, epoxy resins have excellent heat resistance, moldability, and adhesion to a carbon fiber, and are suitable as a sizing agent that provides a carbon fiber reinforced composite material having high mechanical strength. For example, methods in which a carbon fiber bundle is coated with, as a sizing agent, diglycidyl ether of bisphenol A has been proposed (Patent Documents 2 and 3). In addition, methods in which a carbon fiber bundle is coated with an epoxy adduct of a polyalkylene glycol as a sizing agent has been proposed, and it is administered that excellent adhesion is exhibited between the carbon fiber and the matrix resin, and the strength of carbon fiber reinforced composite material is particularly improved (Patent Documents 4, 5, and 6).
On the other hand, application of a sizing agent to a carbon fiber bundle is used also for the purpose of suppressing fluffing due to single yarn breakage in processing process like sheet preparation such as prepreg and molding such as filament winding, and improving processability and handleability. For example, various sizing processing methods have been suggested to thermosetting resin matrices such as epoxy resins, and particularly, sizing agents obtained by adding polyurethane or the like to an epoxy resin as a component for enhancing bundling properties are much suggested (see Patent Documents 7 and 8).
On the other hand, sizing agents having more excellent bundling properties are suggested in so-called short fiber applications like cut yarn and middle yarn. Also, sizing agents for a carbon fiber obtained by blending a polyamide resin (Patent Documents 9 and 10), sizing agents obtained by mixing a polycarbonate resin and a polymaleimide resin and an epoxy resin (Patent Documents 11 and 12) and the like are suggested.